Decanter type centrifugal separator

ABSTRACT

The screw conveyor  200 A includes a first flight  211  and a second flight  212  for transporting cake to cake discharging ports  105  in the small diameter section  103 . Each flight  211, 212  has a small clearance portion  211   a   , 212   a  defining a small clearance d 1  and a large clearance portion  211   b   , 212   b  defining a large clearance d 2  between the tip of the flight and the inner peripheral surface of the small diameter section  103 . The whole region of the large clearance portion  211   b   , 212   b  of each flight  211, 212  overlaps with the small clearance portion  212   a   , 211   a  of another flight  212, 211  at the same point in the direction of the rotation axis C.

TECHNICAL FIELD

The present invention relates to a decanter centrifuge for separatingslurry into liquid and solid cake, and more specifically to a centrifugefor washing the cake effectively.

BACKGROUND OF THE INVENTION

In general, a decanter centrifuge having the capability of draining andwashing is widely known, and called a screen bowl type decantercentrifuge.

Referring now to FIG. 7, the typical screen bowl type decantercentrifuge will be described.

The decanter centrifuge comprises a bowl 100 and a screw conveyor 200rotating in the bowl and rotating relatively thereto.

The bowl 100 comprises a large diameter section 101, a tapered section102, and a small diameter section 103 formed in a single piece. A clearliquid discharging port, or a dam 104 is formed on the end surface ofthe large diameter section 101, and cake discharging ports 105 areformed in the vicinity of end portion of the small diameter section 103.The small diameter section 103 is provided with filtrate dischargingholes 107 formed on its circumferential wall 108. The inner peripheralsurface of the circumferential wall 108 is covered with a porousmaterial 106.

The screw conveyor 200 rotating at a constant differential speed withrespect to the bowl 100 comprises a hub 201 as an axis of rotation and aflight 202 fixed on the hub 201. A washing fluid chamber 203 is providedwithin said hub 201, and washing fluid spray nozzles 204 are provided atthe position corresponding to the washing fluid chamber 203.

Slurry, which is an object to be processed, is supplied to the largediameter section 101 through an unrotatable slurry supply pipe 300within the hub 201. Washing fluid passes through a washing fluid supplypipe 301 provided around the slurry supply pipe 300 and is supplied tothe washing fluid chamber 203 described above through a washing fluidsupply port 302.

Slurry supplied through the slurry supply pipe 300 to the large diametersection 101 is pressed against the inner peripheral surface of the largediameter section 101 by centrifugal force. Liquid component in theslurry is discharged from the dam 104 formed at the end portion of thelarge diameter section 101, and the cake in the slurry is transported bythe flight 202 through the tapered section 102 and the small diametersection 103, and discharged from the cake discharging ports 105. Thecake in the small diameter section 103 is drained while being washed bythe washing fluid.

However, in the related art as described thus far, since the cake isforced to be transported by the flight 202 while being formed intogenerally triangle in cross section with one side situated on thesurface of the flight 202 facing toward the cake discharging ports 105,even when the washing fluid is sprayed onto the portion of generallytriangle in cross section, most part of the washing fluid just flowsover the tilted surface of the cake and does not penetrate into thecake, and thus the effect of the cake washing cannot be expected much.When a large quantity of the washing fluid is supplied to enhance thewashing effect, the amount of waste liquid increases as well, therebyhindering the draining effect. Therefore, the filtering section has tobe extended to produce a satisfactory draining effect, and as aconsequence, the whole length of the apparatus increases, and thus thecost of the apparatus increases as well.

In the related art, an attempt has been made to provide a plate or aknife between the flights at a prescribed distance from the bowl andbreak the accumulation of the cake before spraying the washing fluid inorder to enhance the effect of the cake washing. However, since theseparts may resist transportation of the cake, a larger power is required.In addition, since washing of the machine itself is difficult, theaccumulated cake may cause so called a blockage and thus the object tobe processed cannot be processed in volume stably. In addition, in therelated art described above, the number of components increases and thusthe cost of the apparatus also increases.

DISCLOSURE OF INVENTION

With the problems described above in view, it is an object of thepresent invention to provide a decanter centrifuge that can enhance theeffect of the cake washing without increasing the quantity of thewashing fluid supplied and without increasing the number of components.

In order to achieve the object described above, the decanter centrifugeof this invention comprises:

a bowl;

a screw conveyer provided in the bowl so as to rotate relatively to thebowl;

the bowl comprising a cylindrical large diameter section into whichslurry is supplied, a tapered section having the diameter decreasingfrom the large diameter section, and a small diameter section connectedto the side of the tapered section having a smaller diameter;

the small diameter section being provided with fine filtrate dischargingholes formed on a part or all over the circumferential wall thereof andwith cake discharging ports at the end opposite from the large diametersection;

the small diameter section being provided with washing fluid supplymeans for supplying washing fluid to the cake transported from the largediameter section through the tapered section;

wherein the screw conveyor includes a plurality of flights fortransporting the cake to the cake discharging ports in the smalldiameter section;

the plurality of flights includes a small clearance portion defining asmall clearance and a large clearance portion defining a large clearancebetween the tip of the flight and the inner peripheral surface of thesmall diameter section respectively;

the whole region of the large clearance portion of the plurality of theflights overlaps with the small clearance portion of another flightlocated at the same axial position of the screw conveyer.

The large clearance means a clearance of at least double the smallclearance in size.

In the decanter centrifuge, the washing fluid supply means is preferablyprovided in the large clearance portion on the side facing to the largediameter section. In addition, the washing fluid supply means ispreferably provided so as to be able to spray the washing fluid onto thesurface of the large clearance portion of the flight on the side facingto the large diameter section.

In the decanter centrifuge, the large clearance portion of the flightand the small clearance portion of another flight located at the sameaxial positions are preferably provided in such a manner that when thelarge clearance portion makes at least a half turn, the small clearanceportion passes through the same point.

In addition, in the decanter centrifuge described above, the depth ofthe large clearance and the length of the large clearance portion arepreferably defined in such a manner that cake located at the largeclearance portion on the side facing to the cake discharging portspasses through the large clearance formed between the inner peripheralsurface of the small diameter section and the large clearance portionwhile forming a cake residue layer having the same thickness as thelarge clearance on the side of the large clearance portion facing to thelarge diameter section, and the contact pressure from cake is notapplied to the surface of the large clearance portion of the flightfacing to the cake discharging ports immediately before the largeclearance portion ends and the small clearance portion starts.

As is described thus far, according to the present invention, since alarge clearance portion is formed on the flight, and the washing fluidis supplied onto the thin cake residue layer having passed therethrough,the washing effect can be enhanced without increasing the number ofcomponents or the amount of the washing fluid. Especially, the decantercentrifuge in which the washing fluid is supplied to the surface of thelarge clearance portion of the flight on the side facing to the largediameter section can further enhance the washing effect since thewashing fluid is supplied uniformly onto the whole surface of the thincake residue layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of the decanter centrifuge according tothe first embodiment of the present invention.

FIG. 2 is an explanatory drawing showing the configuration of the flightin the small diameter section according to the first embodiment of thepresent invention.

FIG. 3 is an explanatory drawing showing a state of the caketransportation and a state of the cake washing in a specific region ofthe small diameter section according to the first embodiment of thepresent invention.

FIG. 4 is cross sectional views taken along lines in FIG. 3.

FIG. 5 is an explanatory drawing showing a state of the caketransportation at each position in the small diameter section accordingto the first embodiment of the present invention.

FIG. 6 is an explanatory drawing showing the configuration of the flightin the small diameter section according to the second embodiment of thepresent invention.

FIG. 7 is a cross sectional view of a screen bowl type decantercentrifuge of the related art.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1 to FIG. 6, various embodiments of the decantercentrifuge according to the present invention will be described.

In a first place, a decanter centrifuge according to the firstembodiment of the present invention will be described referring to FIG.1 to FIG. 5.

The decanter centrifuge of this embodiment is basically the same as thedecanter centrifuge of the prior art described referring to FIG. 7, andcomprises a bowl 100 and a screw conveyor 200A (shown in FIG. 1).However, the structure of the flight in the small diameter section 103of the bowl 100 and the mounting position and the mounting direction ofthe washing fluid spray nozzles are different from the centrifugalseparator of the related art. Therefore, identical reference numeralsdesignate portions identical to the centrifugal separator of the relatedart, and the description thereof is omitted. Only the portions differentfrom the related art are described in detail in the following sections.

As shown in FIG. 1, in the small diameter section 103, the hub 201 ofthe screw conveyor 200A is provided with a first flight 211 and a secondflight 212. Each flight 211, 212 comprises small clearance portions 211a, 212 a each defining a small clearance d1 between the tip of theflight and the inner peripheral surface of the small diameter section103, and a large clearance portion 211 b, 212 b each defining a largeclearance d2 therebetween. In FIG. 1, the hollow portions of the flightrepresent the small clearance portions 211 a, 212 a, and the checkeredportions represent the large clearance portions 211 b, 212 b. Thewashing fluid spray nozzles 214 are disposed at the large clearanceportions 211 b, 212 b on the side facing to the large diameter section(the rear side) so as to point the direction in which the washing fluidcan be sprayed onto the surface of the flight facing to the largediameter section.

Referring now to FIG. 2, the small clearance portions 211 a, 212 a, andthe large clearance portions 211 b, 212 b of the flights 211 and 212will be described in detail. In FIG. 2, the circled FIG. 1 designatesthe first flight 211, and the circled FIG. 2 designates the secondflight 212, and dotted lines designate the inner peripheral surface ofthe small diameter section. In the figure (a), a lateral axis representsthe axial position of the flight in the small diameter section, and thevertical axis represents the angle of the flight about the axis ofrotation C, so that the configurations of the flights 211, 212 are shownin this coordinate system. The figure (b) shows the configurations ofthe flights 211, 212 in the direction of the axis of rotation.

The axial dimension of the small diameter section of this embodiment is1000 mm, and the first flight 211 is provided at the intervals of 100 mmalong the whole length of the small diameter section. The second flight212 is provided in a manner that each turn of the second flight isdisposed between each two adjacent turns of the first flight 211 alongalmost the whole length of the small diameter section.

The large clearance portions 211 b, 212 b of the flights 211, 212 areformed at every 495° at the angle width of 270°. The small clearanceportions 211 a, 212 a of the flights 211, 212 are formed at every 270°at the angle width of 495°. In the whole part of the large clearanceportion 211 b, (212 b) of the flight 211, (212), when the largeclearance portion 211 b (212 b) is rotated a half turn (180°), the smallclearance portion 212 a, (211 a) of another flight 212, (211) located atthe same axial position passes the same position. More specifically, thestarting point s and the end point e of the large clearance portion 211b of the first flight 211 overlap with the points s′ and e′ on the smallclearance portion 212 a of the second flight 212 located at the sameaxial position. Therefore, the cake residue layer passed through thelarge clearance d2 between the inner peripheral surface of the smalldiameter section 103 and the large clearance portions 211 b, 212 b ofthe flights 211, 212 and persists thereon is transported to the cakedischarging ports 105 by the small clearance portions 212 a, 211 a ofother flights 212, 211.

For the purpose of illustration, regions of the residue layers of cakeformed on the rear side of all the large clearance portions 211 b, 212 bof the flights 211, 212 are designated to be the first region of thecake residue layer I, the second region of the cake residue layer II, .. . and the seventh region of the cake residue layer VII.

In this embodiment, the washing fluid spray nozzles 214 are not providedfor all the large clearance portions 211 b, 212 b of each flight 211,212 on the side facing to the large diameter section, in other words, itis not provided in all the regions of residue layers of cake, but onlyin the second region of the cake residue layer II and the fourth regionof the cake residue layer IV.

Referring to FIG. 3 and FIG. 4, the state of the cake transportation andthe cake washing in a specific region (in the second region of the cakeresidue layer II and therearound) will now be described. In FIG. 3, thebowl is deployed and the flight is expressed in straight lines in theinterest of clarity of the inner state of the small diameter section.The upper portion of the figure is the direction of the hub of the screwconveyor and the lower portion of the figure is the direction of thecircumference of the small diameter section. FIGS. 4(a), (b), (c), (d),(e) are a cross section taken along the line a—a, a cross section takenalong the line b—b, a cross section taken along the line c—c, the crosssection taken along the line d—d, and the cross section taken along theline e—e respectively in FIG. 3. In FIG. 3 and FIG. 4, a very thin cakeresidue layer formed after the cake has passed through the clearancebetween the small clearance portions 211 a, 212 a of the flight and theinner peripheral surface of the small diameter section is omitted.

As shown in FIG. 3 and FIG. 4(a), in the process that the smallclearance portion 212 a of the second flight 212 moves toward the cakedischarging ports, the cake C1 is pressed by the surface of the smallclearance portion 212 a of the flight on the side facing to the cakedischarging ports and transported while being formed into generallytriangle in cross section with one side situated on the surface of theflight, as mentioned in relation with the prior art.

As shown in FIG. 3 and FIGS. 4(b), (c), when the cake reaches the largeclearance portion 212 b of the second flight 212, the cake just passesthrough the large clearance d2 formed between the large clearanceportion 212 b and the inner peripheral surface of the small diametersection 103, and is not transported. As a consequence, a cake residuelayer C2 is formed behind the large clearance portion 212 b, in otherwords, in the second region of the cake residue layer II. Therefore, thequantity of the cake on the side of the cake discharging side (frontside) of the large clearance portion 212 b decreases gradually from thestarting point s toward the end point e, and the cake to be transporteddisappears at the end point e. The cake residue layer C2 formed on therear side of the large clearance portion 212 b has a thicknesscorresponding to the depth of the large clearance d2.

The washing fluid from the washing fluid spray nozzles 214 are suppliedonto the flight surface on the rear side of the large clearance portion212 b and the portion in which residue layers of cake C2 start to beformed. In other words, the washing fluid is supplied to the cakeresidue layer C2 successively in the process that cake passes from thelarge clearance portion 212 b toward the rear and the cake residue layerC2 starts to be formed. Therefore, the washing fluid is applieduniformly on the whole surface of the cake residue layer C. The washingfluid applied on the portion in which the cake residue layer C2 startsto be formed penetrates into the cake layer in a very short time bycentrifugal force generated by the rotation of the bowl 100 and movesinto the porous material 106 while dissolving soluble component(impurities) in the cake layer, most part of which passes through theporous material 106 and discharged out of the machine through thefiltrate discharging holes 107.

In this way, according to the present embodiment, since the washingfluid is supplied to the surface of the cake residue layer C2 that isgenerally vertical with respect to centrifugal force generated by therotation of the bowl 100, the washing fluid does not flow on the surfaceof the cake, but most part of the washing fluid penetrates into thecake. Since the thickness of the cake residue layer C2 is smaller thanthat of the cake C1 of generally triangle in cross section, most part ofthe washing fluid applied to the cake residue layer C2 passes throughthe cake residue layer C2 and then through the porous material 106 andthe filtrate discharging holes 107 and is discharged out of the machinealmost without fail. In addition, according to this embodiment, thewashing fluid is uniformly supplied on the whole surface of the cakeresidue layer C as described above. Therefore, according to thisembodiment, a single washing operation produces a satisfactory anduniform washing effect for all the cake residue layer C2.

Since centrifugal force is always acting on the cake, a drainingoperation is always on the second region of the cake residue layer II.Therefore, according to this embodiment, since the draining operationacts uniformly upon the spread cake in the second region of the cakeresidue layer II, a satisfactory draining effect is also obtained.

The cake residue layer C2 passed through the large clearance portion 212b of the second flight 212 is scraped and collected sequentially by thesmall clearance portion 211 a of the first flight 211 positioned behindthe large clearance portion 212 b of the second flight 212, as shown inFIG. 3 and FIGS. 4(c) and (d), and thus the cake is accumulatedgradually in front of the small clearance portion 211 a, and again, asshown in FIG. 3 and FIG. 4(e), a cake layer C1 of generally triangle incross section is formed. The cake layer C1 is transported toward thecake discharging ports by the small clearance portion 211 a of the firstflight 211, and when it reaches the large clearance portion 211 b of thefirst flight 211, it passes therethrough and forms again a cake residuelayer C2 at the rear, in other words, at the third region of the cakeresidue layer III. In this way, in this embodiment, the washing effectand the draining effect are enhanced since the cake is broken andagitated every time when the cake is formed into generally triangularshapes in cross section and then into layers repetitively.

The cake is then continued to be processed in the same manner from thefourth region of the cake residue layer IV, . . . , to the seventhregion of the cake residue layer VII, and finally transported to thecake discharging ports 105 and discharged therethrough to the outside.In this embodiment however, since the washing fluid spray nozzles 214are provided only at the second region of the cake residue layer II andthe fourth region of the cake residue layer IV, no washing fluid issupplied to the cake residue layer C2 at the third region of the cakeresidue layer III, the fifth region of the cake residue layer V, thesixth region of the cake residue layer VI, and the seventh region of thecake residue layer VII.

The reason why the washing fluid is supplied to the cake residue layeronly at the second region of the cake residue layer II and the fourthregion of the cake residue layer IV in this embodiment is that cakeprocessed here is relatively low in fluid penetrating property. When thefluid penetrating property of the cake is low, even when the washingfluid is supplied in the second region of the cake residue layer,drainage cannot be completed in the second region of the cake residuelayer. Therefore, when the washing fluid is supplied again in the thirdregion of the cake residue layer, the washing fluid may stay on the cakelayer and may result in lowering of the washing and draining effectinstead. In contrast to it, when processing cake having a good fluidpenetrating property, it is recommended to supply the washing fluid inthe first region of the cake residue layer I, in the second region ofthe cake residue layer II, and in the third region of the cake residuelayer III consecutively.

In this embodiment, it seems that forming the cake residue layer in thefifth, sixth, and seventh regions of residue layers of cake ismeaningless because no washing fluid is supplied in the regions forwardto the cake residue layer IV. However, it contributes to enhance thedraining effect by applying centrifugal force repeatedly to the cakespread in layers.

Referring now to FIG. 5, the conditions of the cake transportation ateach axial position will be described.

When the cake is transported to the first region of the cake residuelayer I by the small clearance portion 211 a of the first flight 211,the first flight 211 is transitioned from the small clearance portion211 a to the large clearance portion 211 b, and thus the cake residuelayer C2 remains behind the large clearance portion 211 b of the firstflight 211 in the first region for forming the cake residue layer I. Thecake residue layer C2 is scraped by the small clearance portion 212 a ofthe second flight 212 that passes the first region for forming the cakeresidue layer I a half turn (180°) behind the large clearance portion211 b of the first flight 211 and transported to the second region forforming the cake residue layer II. The second flight 212 transitionsfrom the small clearance portion 212 a to the large clearance portion212 b when it reaches the second region for forming the cake residuelayer II. Therefore, the cake residue layer C2 remains behind the largeclearance portion 212 b of the second flight 212 in the second regionfor forming the cake residue layer II, so that the washing fluid issupplied to the cake residue layer C2.

The cake residue layer C2 is scraped by the small clearance portion 211a of the first flight 211 passing a half turn (180°) behind the largeclearance portion 212 b of the second flight 212 through the secondregion for forming a cake residue layer II and transported to the thirdregion for forming the cake residue layer III.

Likewise, the same process is repeated until the cake is transported tothe cake discharging ports 105.

As is described thus far, in this embodiment, since the configuration ofthe flight in the small diameter section 103 and the position andorientation of the washing fluid spray nozzles 214 are modified so thatthe cake is spread out into a layer in the process of transporting cakein the small diameter section 103 and the washing fluid is supplieduniformly thereon, the washing effect and draining effect for the cakecan be enhanced with very little increase of the cost of the apparatus.

The inventor manufactured a test machine of the decanter centrifugeaccording to this embodiment and conducted a test on this test machinewith slurries of gypsum powder, terephthalic acid powder, or pulverizedpolyethylene terephthalate resin and so on dispersed in water withacetic acid added as an impurity. As a result it has been shown that thewashing effect and the draining effect were satisfactory for all theobjects to be processed which were subjected to the test.

In order to enhance the effect of the cake washing, as shown in FIG.4(d), it is important that no cake remains in front of the largeclearance portion 212 b immediately before the large clearance portion212 b ends, in other words, a cake residue layer C2 having the samecross sectional area as the cake C1 of the same figure (a) is formedbehind the large clearance portion 212 b.

It is because if any cake remains in front of the large clearanceportion 212 b immediately before the large clearance portion 212 b endsas shown in the same figure (d′), the cake C3 is transported by thesmall clearance portion without having supplied this cake C3 with thewashing fluid. Therefore, it is preferable that the depth of the largeclearance d2 and the length of the large clearance portions 211 b, 212 bare determined so that almost no cake remains downstream the largeclearance portions 211 b, 212 b immediately before they are ended, inother words, so that no pressure from the cake is applied to the frontsurface of the flight at the large clearance portions 211 b, 212 b.

Now, specific depths of the small clearance d1 and the large clearanced2 will be described.

Assuming that the design conditions of the decanter centrifuge of thisembodiment are as follows;

inner diameter of the small diameter section: 260 mm

length of the small diameter section: 1000 mm

pitch of the conveyor: 100 mm

differential speed of the conveyor: 40 rpm

amount of cake to be discharged: 18.3 litters/minute,

Under the above-described conditions, the triangular cross sectionalarea of the cake C1 in FIG. 4(a) is approximately 5.6 cm². In order thatall of the cake C1 is formed into the cake residue layer C2 as shown inFIG. 4(d), when the length of the large clearance portions 211 b, 212 bcorresponds 360° as a simple example, the depth of the large clearanced2 has to be at least the depth of the small clearance d1 plus 5.6 mm.Since the length of the large clearance portions 211 b, 212 b of thisembodiment is as long as the length corresponding to 270°, the depth ofthe large clearance d2 of this embodiment has to be at least the depthof the small clearance d1 plus 7.5 mm (=5.6 mm×360°/270°). However,since a loss of the cake during transportation by the flight, or a lossor a swell of the cake due to a resistance generated when being passedthrough the large clearance or due to slippage or drainage when beingsupplied with the washing fluid have to be considered in actual fact,the depth of the large clearance d2 is preferably determined to be 10 to20% larger than the calculated value.

On the other hand, the small clearance d1 is preferably determined assmall as possible, and thus it is normally set to 0.5 mm to 1.5 mmconsidering error in manufacturing of the machine or warpage of thescrew conveyor.

In this embodiment, with these conditions in view, the depth of thesmall clearance d1 is set to 1.0 mm and the depth of the large clearanced2 is set to 9.6 mm (=1.0 mm+7.5 mm×1.15).

Referring now to FIG. 6, the second embodiment of the decantercentrifuge according to the present invention will be described. In FIG.6 as well as in FIG. 2(a), the lateral axis represents the axialposition of the small diameter section and the vertical axis representsthe angle about the axis of rotation so as to show the configuration ofeach flight in this coordinate system.

The circled figures designate the number of the flight respectively.

As shown in FIG. 6, the small diameter section is provided with morethan four flights in this embodiment. The flights 221, 222, 223, 224have small clearance portions 221 a, 222 a, 223 a, 224 a and largeclearance portions 221 b, 222 b, 223 b, 224 b respectively formedthereon. The whole region of the large clearance portions 221 b, 222 b,223 b, 224 b of the flights 221, 222, 223, 224 is constructed in such amanner that when the large clearance portion makes a ⅔ turn (240°), thesmall clearance portion of another flight located at the same axialposition passes through the same point.

In this way, even when three or more flights are provided in the smalldiameter section, basically the same effect as the first embodiment canbe obtained. In this embodiment, the whole region of the large clearanceportion of the flight is constructed in such a manner that when thelarge clearance portion makes a ⅔ turn (240°), the small clearanceportion of another flight located at the same axial position passesthrough the same point. As the result of this, the period of time untila cake residue layer is scraped out by the small clearance portion ofthe next flight increases and more washing fluid is removed through theouter periphery of the small diameter section, thereby further enhancingthe washing effect and the draining effect in comparison with the firstembodiment.

What is claimed is:
 1. A decanter centrifuge comprising: a bowl; a screwconveyer provided in said bowl so as to rotate relatively to said bowl;said bowl comprising a cylindrical large diameter section into whichslurry is supplied, a tapered section having the diameter decreasingfrom said large diameter section, and a small diameter section connectedto the side of said tapered section having a smaller diameter; saidsmall diameter section having fine filtrate discharging holes formed ona part or all over the circumferential wall thereof and cake dischargingports formed at an end opposite from said large diameter section; saidsmall diameter section is provided with washing fluid supply means forsupplying washing fluid to cake transported from said large diametersection through said tapered section; wherein said screw conveyorincludes a plurality of flights for transporting said cake to said cakedischarging ports in said small diameter section; said plurality offlights include a small clearance portion defining a small clearance anda large clearance portion defining a large clearance between the tip ofsaid plurality of flights and the inner peripheral surface of said smalldiameter section respectively; the whole region of said large clearanceportion of said plurality of flights overlaps with said small clearanceportion of another flight located at the same axial position of saidscrew conveyer.
 2. A decanter centrifuge as set forth in claim 1,wherein said large clearance portion of said flight and said smallclearance portion of another flight located at the same axial positionare provided in such a manner that when said large clearance portionmakes at least a half turn, said small clearance portion passes throughthe same point as said large clearance portion.
 3. A decanter centrifugeas set forth in claim 1, wherein a depth of said large clearance and alength of said large clearance portion are determined in such a mannerthat said cake located at said large clearance portion on the sidefacing to said cake discharging ports passes through said largeclearance formed between an inner peripheral surface of said smalldiameter section and the said large clearance portion while forming acake residue layer substantially having the same thickness as said largeclearance on the side of said large clearance portion facing to saidlarge diameter section, and no contact pressure from said cake isapplied to the surface of said large clearance portion of the flightfacing to said cake discharging ports immediately before said largeclearance portion ends and said small clearance portion starts.
 4. Adecanter centrifuge as set forth in any one of claims 1, 2 and 3,wherein said washing fluid supply means is provided in said largeclearance portion on the side facing to said large diameter section. 5.A decanter centrifuge as set forth in claim 4, wherein said washingfluid supply means is provided so as to be able to spray said washingfluid onto the surface of said large clearance portion of the flight onthe side facing to said large diameter section.